Tunnel-type giant magnetoresisitance in Co-Al-Ta-O insulated granular system

We have investigated the tunnel-type giant magnetoresistance (TMR), the related magnetic and electric properties and the microstructure of Co-Al-Ta-O granular thin films which were prepared by the reactive sputter-deposition technique with oxygen and argon. The TMR attained a maximum value of 12.5% at room temperature under a magnetic field of 1 T for Co₄₀Al₁₃Ta₃O₄₄. The MR ratio is the largest in a granular system similar to Co-Al-O. During the sputtering, O₂ gas flow was introduced at 2.2sccm into the chamber filled with Ar at a total pressure higher than 1 × 10^-4 Torr. The specific electric resistance of the film that showed the maximum TMR is 1.46 × 10⁵ μΩ·cm. Ta forms an oxide compound similar to Ta₂O₅ in the insulator matrix. The effect of Ta addition on the TMR appears at around 2.8 at.%. The MR ratio increases with an increase of the fraction of fcc-Co, accompanying the decrease of the Co-O fraction and the decrease in ferromagnetic interaction between Co particles. The specimen that shows large TMR consists of nanometer-sized Co particles which are completely isolated by amorphous Al₂O₃ and Ta₂O₅ intergranular layers. The standard free energy of oxidation of Co, Al and Ta may play an important role in the formation of the microstructure of Co-Al-Ta-O granular films and in TRM improvement.